The invention relates to detection of small concentrations of trace gases in a carrier gas, and, in particular, to apparatus for detecting the loading of air with war gases.
U.S. Pat. No. 3,558,283 describes a method of determining the reactive hydrocarbon content of automotive or exhaust gases, whereat a stream of said gases is brought into contact with electrically heated combustion filaments in opposed arms of a Wheatstone bridge. Afterwards the reactive hydrocarbons are removed from the stream by passing that stream through a stripping column and then passing this stream over the opposed other arms of the same bridge and applying the output of the Wheatstone bridge to provide a signal corresponding to the amount of reactive hydrocarbons in the exhaust gases. In a second embodiment shown in the same U.S. patent the gas stream is split into two equal portions, whereat one of them is subject to combustion. The reactive hydrocarbons are removed from the other of said portions by passing it through a stripping column and afterwards subjecting said other portion also to combustion. Finally the heat effects of combustion of the two portions are converted into electrical signals and by means of a differential amplifier are converted to an electrical signal corresponding to the amount of reactive hydrocarbons in the exhaust gases.
The first method comprising the Wheatstone bridge arrangement requires four separate measuring chambers and therewith is complicated. The second version strongly depends on both portions of the gas stream being maintained equal. The same applies to other measuring apparatus using two parallel gas streams, such as the apparatuses described in U.S. Pat. Nos. 3,997,297 and 3,620,931.
U.S. Pat. No. 3,835,328 describes a baffle cell measuring apparatus for detecting small concentrations of trace gases, where a gas mixture containing said trace gases flows through a cell containing a radiation source for ionizing the gas molecules, a recombination zone consisting of baffles or which provide a labyrinth-like flow path of increased length and a collector electrode at the outlet of said recombination zone. Further improvements of this ionization detector are described in U.S. Pat. Nos. 4,075,550, 4,238,678 and 4,362,941. In all those known baffle cells an airstream is generated through the cell by means of a ventilator or a pump. At the input of the cell an ionization means such as a radiation source is provided and the gas stream flows through a flow path, the length of which is increased in the form of a labyrinth. Within this labyrinth path most of the ionized carrier gas molecules recombine, whereat the trace gases together with ions of the carrier gas form molecule packets or clusters which do not recombine within said labyrinth. These clusters rather move to the collector electrode and there generate a corresponding electrical current.
Problems arise if the trace gas such as mustard gas is present only in very small concentrations and therefor the generated electrical currents are only available in the range of 10.sup.-12 A (10.sup.-12 A=1pA). In those cases the measuring signal might be covered by the noise of the subsequent amplifier. Furthermore, one cannot be sure that all ionized carrier gas molecules really recombine so that by non-recombined carrier gas molecules a further interference signal may be generated. Particular difficulties arise if the detector has to be used in a large temperature range such as from -30.degree. C. to +50.degree. C. and is expected to measure accurately over the entire range. Furthermore, it is desired that changes of the supply voltage or any external electromagnetic radiation should not influence the measuring result.